The present invention relates to a system for monitoring pressure of tires in a wheeled vehicle. In particular, the invention relates to a tire pressure monitoring system employing individual battery-powered pressure sensors in each wheel, for transmitting uniquely coded information to a receiver mounted on the vehicle for display to a vehicle operator.
The art is replete with various known approaches to providing indications of tire pressure. The approaches include tire pressure sensors which provide temperature-compensated tire pressure information. Compensation for changes in temperature is important. Because air expands with temperature, and because tires get hotter the longer a vehicle is operated, failure to provide compensation for temperature can yield inordinately high pressure readings. Likewise, particularly cold weather can yield low tire pressure readings. These high and low readings need to be normalized to a constant temperature.
For the most part, known approaches to temperature compensation have employed electronic alteration of tire pressure sensor outputs. Examples may be found in U.S. Pat. Nos. 4,567,459, 4,703,650, and 4,966,034.
The art further includes a number of known approaches for communicating tire pressure information to a vehicle operator inside the vehicle. These approaches include tuned circuits and radio transmitters. Examples of the former may be found in the just-mentioned U.S. patents. Examples of radio transmitter approaches may be found in U.S. Pat. Nos. 4,510,484, 4,554,527, and 5,061,917.
Another aspect of known tire pressure monitoring systems relates to the coding of digital values associated with respective tires on a vehicle. Examples of such approaches are found in U.S. Pat. No. 5,001,457 and in just-mentioned U.S. Pat. No. 5,061,917.
Integrated circuit technology and power generation technology have progressed to the point where long-life batteries can power small integrated circuits. An example of a system employing battery-powered transmitters on vehicle wheels is found in U.S. Pat. No. 4,978,941.
Known remote tire pressure monitoring systems are deficient in several respects. First, while presumably some of these systems are calibrated somehow during vehicle assembly to enable proper display on, for example, a dashboard console, these systems do not address the problem arising from the need to recalibrate the systems when the vehicles undergo standard maintenance such as tire rotation. The problem of replacement of damaged transmitters, and of calibration of the replacements, does not appear to be addressed, either.
Another deficiency relates to the coding of transmitters for associated receivers. As the cost of assembly of such systems decreases, the systems will proliferate. If there is overlapping of codes among different systems, false pressure readings from tires of one vehicle could be reported on the console of another vehicle in close proximity, having been received erroneously by a receiver on that other vehicle.
Also, while attention has been paid in the past to selection of appropriate transmission frequencies to avoid interference from other radio frequency sources, particular problems are presented by remote control systems, such as keyless entry systems, which are becoming more and more widely implemented.
Yet another problem which does not seem to have been addressed in known systems relates to battery drain in transmitters prior to installation, and avoidance of excessive battery drain in operation, in the absence of a low pressure reading.
It would be desirable to provide a tire pressure monitoring system which can learn, or relearn, tire locations simply on a vehicle when tires are rotated or replaced. It also would be desirable to provide a system which is not affected by similar systems on other vehicles, or by other radio frequency generating systems within the vehicle, such as keyless entry systems. Further, it would be desirable to provide a tire pressure monitoring system which takes advantage of newer longer-life power sources, and which uses temperature-compensated pressure sensor technology which is not power dependent in normal operation, so as to conserve power.